Whenever the display is sleeping (the laptop is still awake, just the display is off), and when it wakes up again (by moving the mouse, or issuing a keypress); then the backlight setting is at 100% regardless of what it was set at before the display went to sleep.

If I press the Fn+F8 key to set it again, one press is enough for the backlight level to go down several increments; back to where it was before the display turned off.

This didn't happen with the same Ubuntu install but running the stock BIOS.

For context, here is an excerpt from the mailing list:

this is a known issue. It actually was there all the time but only
got more visible with the introduction of ACPI/OpRegion for the inte-
grated GFX. Here is what I found out so far (on my ArchLinux):
w/o OpRegion:
1. `acpi_video` driver gets loaded, systemd restores brightness from
last shutdown.
2. `i915` driver gets loaded, reads the current brightness.
3. `i915` exposes backlight as `intel_backlight`, systemd restores
brightness from last shutdown here as well.
Every time `i915` power cycles the backlight, it restores the value it
has seen when it was loaded.
w/ Opregion the order somehow changes to 2. 1. 3., now `i915` always
restores to 100% that it read before systemd restored the brightness.
A sane solution would be to extend XBCM in `src/drivers/intel/gma/acpi/
configure_brightness_levels.asl` to always notify `i915` when we change
the backlight through OpRegion mailbox 3.

Happy to provide more logs or details as required. Thanks!

History

This also occurs for me on a T420 running Debian 9.4. Has there been any progress? Is there anything I/we can do to help? I do not have coding knowledge, so my ability to assist will be limited, but I am willing to try.